Rotating three dimensional surveillance radars normally use a template or fan of search beams which cover the elevation extent of the surveillance volume and are steered to the mechanical azimuth of the antenna. This approach provides a uniform distribution of radar energy and probability of detection as a function of azimuth when presented with azimuthally symmetric environmental conditions. If however, the environmental conditions are not azimuthally symmetric, such as in the presence of locally strong clutter or weather conditions at particular azimuth angles, then this scanning approach results in locally reduced probability of detection and target range at those angles. Rain interference in the microwave region can be of such intensity that the tracking of an aircraft, or for that matter any object, becomes difficult, if not impossible.
To compensate for these types of conditions, low frequency bands have been used because precipitation and cloud losses are significantly lower with lower frequency (e.g. Ultra High Frequency (UHF), Very High Frequency (VHF), L band, S band). However, the amount of energy spent per unit azimuth in a rotating three dimensional (3D) surveillance radars is normally limited by the rotation rate, e.g., for a 12 RPM antenna, 13.8 milliseconds of radar resources are available for each 1 degree of azimuth during rotation.
If a heavy rain cell exists in an isolated azimuth region, the target SNR decreases and reduces the probability of detection (Pd) at that azimuth. Assuming a duty constraint, simply using a higher energy template to counter the extra loss is not effective. More energy increases template duration and thus azimuth spacing. However, increased azimuth spacing increases beamshape loss and increased beamshape loss counteracts the increased energy applied. The result is that target detection range decreases significantly at the azimuth angles with the storm cell while remaining high elsewhere.
Existing rotating 3D surveillance radar designs use a static azimuth scan angle off broadside and electronically scan in elevation. Other optional radar design tradeoffs may consider reduction of search elevation extent to allow more energy per steradian per unit of time at azimuth angles where degraded conditions exist. Further, designing system sensitivity for worst case storm azimuth results it excess margin elsewhere, i.e., increases cost. In addition, limiting the maximum elevation in heavy rain so that more energy can be scheduled at low elevations cuts the search volume.